1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal molecules is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
While various types of liquid crystal display devices are known, active matrix LCDs having thin film transistors and pixel electrodes arranged in a matrix are probably the most common. This is because such active matrix LCDs can produce high quality images at reasonable cost.
Recently, liquid crystal display (LCD) devices with light, thin, and low power consumption characteristics are used in office automation equipment and video units and the like. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode. Although TN-LCDs and STN-LCDs have been put to practical use, they have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and liquid crystals interposed between the upper and lower substrates.
A detailed explanation about operation modes of a typical IPS-LCD panel will be provided referring to FIGS. 1 to 3.
As shown in FIG. 1, upper and lower substrates 1 and 2 are spaced apart from each other, and a liquid crystal layer 3 is interposed therebetween. The upper and lower substrates 1 and 2 are called color filter substrate and array substrate, respectively. Pixel and common electrodes 4 and 5 are disposed on the lower substrate 2. The pixel and common electrodes 4 and 5 are parallel with and spaced apart from each other. The pixel and common electrodes 4 and 5 apply a horizontal electric field 6 to the liquid crystal layer 3. The liquid crystal layer 3 has a negative or positive dielectric anisotropy, and thus it is aligned parallel with or perpendicular to the horizontal electric field 6, respectively.
FIGS. 2A and 2B conceptually illustrate operation modes of a conventional IPS-LCD device. When there is no electric field between the pixel and common electrodes 4 and 5, as shown in FIG. 2A, the long axes of the liquid crystal molecules maintain an angle from a line perpendicular to the parallel pixel and common electrodes 4 and 5. Herein, the angle is 45 degrees, for example.
On the contrary, when there is an electric field between the pixel and common electrodes 4 and 5, as shown FIG. 2B, there is an in-plane horizontal electric field 6 parallel with the surface of the lower substrate 2 between the pixel and common electrodes 4 and 5. The in-plane horizontal electric field 6 is parallel with the surface of the lower substrate 2 because the pixel and common electrodes 4 and 5 are formed on the lower substrate 2. Accordingly, the liquid crystal molecules are twisted so as to align, for example, the long axes thereof with the direction of the horizontal electric field 6, thereby the liquid crystal molecules are aligned such that the long axes thereof are parallel with the line perpendicular to the pixel and common electrodes 4 and 5.
By the above-mentioned operation modes and with additional parts such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has wide viewing angles since the pixel and common electrodes are together placed on the lower substrate. Moreover, the fabricating processes of this IPS-LCD device are simpler than those of other various LCD devices.
However in the IPS-LCD device, a color-shift which depends on the viewing angle still remains. It is already known that this color-shift cannot be acceptable for full color-image display. This color-shift is related to a rotational direction of the liquid crystal molecules under application of electric field when the applied voltage is greater than the threshold voltage. Moreover, this color-shift is caused by increasing or decreasing of an optical retardation (xcex94nxc2x7d) of the liquid crystal layer with viewing angle.
For the sake of discussing the above-mentioned problem of the IPS-LCD device, with reference to FIG. 3, the specific pixel structure of the IPS-LCD device is employed and will be described in detail.
As shown in FIG. 3, the pixel and common electrodes 7 and 8 have bend angle xcex1. These bend electrode""s structure allows the liquid crystal molecules 9 to rotate in opposite direction in each pixel when the voltage is supplied to the bend electrodes. Therefore, the bend electrodes 7 and 8 and the oppositely directed liquid crystal molecules 9 divide the pixel into two different regions with different viewing angle characteristics. And thus, the color-shift can be effectively compensated by this multi domain structure.
However, when the voltage is turned ON, extraordinary domains appear around the bottom edges of driving electrodes. These extraordinary domains degrade the picture quality and reliability of the IPS-LCD device having the bend electrodes. Namely, disclination appears at the edges of the pixel areas, and thus this disclination manifests as positional non-uniformities in the transmittance of light.
FIGS. 4A and 4B are enlarged partial plan views of pixel and common electrodes. These figures illustrate arrangement of the liquid crystal molecules and the electric field when the voltage is turned ON. As shown, a common electrode 11 is extended from a common line 23, and a pixel electrode 21 is disposed parallel with the common electrode 11. The common electrode 11 forms an acute angle with the common line 23 as depicted in a portion xe2x80x9cAxe2x80x9d of FIG. 4A while the pixel electrode 21 forms an obtuse angle with the common line 23 as shown in a portion xe2x80x9cDxe2x80x9d of FIG. 4A. When the voltage is supplied to the common and pixel electrodes 11 and 21, the electric field occurs between the common and pixel electrodes 11 and 21. However at this time, a distortion of the electric field appears around the acute and obtuse angels, the portions xe2x80x9cAxe2x80x9d and xe2x80x9cDxe2x80x9d. Thereupon, reverse rotational deformation is caused by this distortion of the electric field around the portions xe2x80x9cAxe2x80x9d and xe2x80x9cDxe2x80x9d.
Referring to FIG. 4B, when the voltage is applied to the pair of electrodes 11 and 21, the liquid crystal molecule 41 located in the parallel electric field area turns clockwise while the liquid crystal molecule 51 located in the distorted electric field area turns counterclockwise. So the orientation direction of the liquid crystal is different between the parallel electric field area and the distorted electric field area, and thus the disclination occurs in the distorted electric field area. This disclination causes a decrease in the aperture ratio, and a change of the orientation direction causes traces of the extraordinary domains. These features also affect response characteristic of the liquid crystal layer, and an afterimage phenomenon occurs in the display area
Accordingly, the present invention is directed to an IPS-LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an array substrate for use in the IPS-LCD device having an increased aperture ratio.
Another object of the present invention is to provide the array substrate for use in the IPS-LCD device which suppresses the traces of extraordinary domains and afterimage phenomenon.
Another object of the present invention is to provide the array substrate for use in the IPS-LCD device which improves the response characteristics of the liquid crystal layer.
In order to achieve the above object, the first preferred embodiment of the present invention provides an array substrate for use in an in-plane switching liquid crystal display device including a plurality of gate lines on a substrate; a plurality of data lines over the substrate, each data line being perpendicular to each gate line; a common line on the substrate, the common line being parallel with and spaced apart from the gate line; a plurality of common electrodes extended from the common line and elongated along the data line, wherein each common electrode has a plurality of bend portions, and wherein each common electrode has a sawtooth-shaped base in contacting part where each common electrode meets the common line in order to form an obtuse angle with the common line; a plurality of pixel electrodes spaced apart from and elongated along the common electrodes, wherein each pixel electrode has a plurality of bend portions and corresponds to each common electrode; a connecting line contacting one end of each pixel electrode, the connecting line electrically connecting pixel electrodes; a switching element electrically connected with the gate and data lines, the switching element supplying voltage to the pixel electrodes.
Each pixel electrode has a sawtooth-shaped base in contacting part where each pixel electrode meets the connecting line, and makes an obtuse angle with the connecting line using the sawtooth-shaped base.
The connecting line overlaps a portion of each gate line and comprises a storage capacitor with each gate line. One of the common electrodes elongates along the data line and electrically communicates with adjacent pixels.
The common line crosses the one of bend portions of each common electrode, and electrically connects a plurality of common electrodes. Moreover, the common line elongates along the gate line and communicates with the other common lines that are located in the adjacent pixels.
The present invention also provides, in another aspect, an array substrate for use in an in-plane switching liquid crystal display device including a gate line on a substrate; a data line over the substrate, the data line being perpendicular to the gate line; a common line being parallel with and spaced apart from the gate line; a plurality of common electrodes extended from the common line, wherein each common electrode has a zigzag shape and a sawtooth-shaped base, and wherein each common line forms an angle of greater than 90xc2x0 with the sawtooth-shaped base; a connecting line being parallel with the gate line; a plurality of pixel electrodes extended from the connecting line, wherein each pixel electrode has a zigzag shape and a sawtooth-shaped base, and wherein each common line forms an angle of greater than 90xc2x0 with the sawtooth-shaped base; and a switching element electrically connected with the gate and data lines, the switching element supplying voltage to the pixel electrodes.
The aforementioned switching element is located in the crossing of the gate and data lines. This switching element includes a source electrode that is extended from the data line; a gate electrode that is extended from the gate line; a drain electrode that contacts one of the pixel electrodes through a drain contact hole; an active layer that is formed over the gate electrode and between the source and drain electrodes; and ohmic contact layers that are formed between the active layer and the source and drain electrodes.
One of the pixel electrodes has a bend end portion over the drain electrode. This bend end portion overlaps one end of the drain electrode and contacts the drain electrode through the drain contact hole
The connecting line overlaps a portion of the gate line, and the connecting line and the gate line comprise a storage capacitor. A plurality of the pixel electrodes and the connecting line can be made of a transparent conductive material. However, a plurality of the pixel electrodes and the connecting line can be made of an opaque metallic material.
A plurality of the common electrodes and the common line can be made of a transparent conductive material. However, the plurality of the common electrodes and the common line can be made of an opaque metallic material.
The present invention also provides, in another aspect, an array substrate for use in an in-plane switching liquid crystal display device including a gate line on a substrate; a data line over the substrate, the data line being perpendicular to the gate line, wherein each pair of gate and data lines defines a pixel area; a common line being parallel with and spaced apart from the gate line, wherein the common line is located in any region of the pixel area and elongates along the gate line; a plurality of common electrodes extended from the common line, wherein each common electrode has a zigzag shape and sawtooth-shaped base in the intersection where each common electrode crosses the common line, wherein each common line forms an angle of greater than 90xc2x0 with the sawtooth-shaped base, and wherein one of the common electrodes elongates along the data line; a connecting line being parallel with the gate line; a plurality of pixel electrodes extended from the connecting line, wherein each pixel electrode has a zigzag shape and a sawtooth-shaped base, and wherein each common line forms an angle of greater than 90xc2x0 with the sawtooth-shaped base; and a switching element electrically connected with the gate and data lines, the switching element supplying voltage to the pixel electrodes.
One of the pixel electrodes has a sharply bent end portion over the switching element that is located in the crossing of the gate and data lines. This switching element includes a source electrode that is extended from the data line; a gate electrode that is extended from the gate line; a drain electrode that is the bent end portion of one pixel electrode; an active layer that is formed over the gate electrode and between the source and drain electrodes; and ohmic contact layers that are formed between the active layer and the source and drain electrodes.
The drain electrode and the pixel electrodes can be separately formed on a different layers. Moreover, a substance of which the drain electrode is made can be different from that of the pixel electrodes. However, the data line, the connecting line, the pixel electrodes, and the source and drain electrodes can be made of the same material.
The connecting line overlaps a portion of the gate line, and the connecting line and the gate line comprise a storage capacitor. And the common line and each common electrode intersect in one bend portion of each common electrode. Moreover, the common line is connected with the other common lines that are located in the adjacent pixel areas in order to form a mesh shape.
One of the common electrodes is connected with the other common electrodes that are positioned in the adjacent pixel areas in order to form the mesh shape.
A plurality of the pixel electrodes and the connecting line can be made of a transparent conductive material. However, the plurality of the pixel electrodes and the connecting line can be made of an opaque metallic material.
A plurality of the common electrodes and the common line are made of a transparent conductive material. However, the plurality of the common electrodes and the common line are made of an opaque metallic material.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.